J.L. Otegui

Numerical and experimental determination of three-dimensional multiple crack growth in fatigue

Abstract This work is concerned with the assessment of propagation of multiple fatigue cracks in three-dimensions. Computational modelling of fatigue crack propagation is made together with detection and monitoring of the crack shape development. The boundary element method (BEM) is used for automating the modelling of crack propagation in linear elastic as well as elastic–plastic regimes. Strain at several positions on the specimen surface near the crack mouth is measured to monitor crack initiation, shape development and closure levels. Examples are provided to validate the model by comparing the experimental results with those obtained by numerical predictions.

Weld failures in sleeve reinforcements of pipelines

Abstract Based on fractographic, metallurgical, mechanical and fracture mechanics analyses, the causes of three failures in welded full encirclement sleeve repairs in a 24 in gas pipeline were evaluated. These failures were related to poor manufacturing procedures. The material used to build the sleeves was old and had poor transverse strength. High heat input cellulosic electrodes were used to weld the field joints, which lead to hydrogen embrittlement in the HAZ, also helped by relatively high circumferential stresses and defects of lack of fusion. A series of changes were introduced, including improvements in the in-plant fabrication and the in-field installation of the repair sleeves. A low hydrogen weld procedure with controlled penetration, NDE specifications and epoxy fillers were introduced to minimize the risk of sleeve failures and plastic collapse of the pipe.

Influence of multiple sleeve repairs on the structural integrity of gas pipelines

Abstract This paper addresses the structural integrity of gas pipelines with multiple full-encirclement weld repairs. The scope of the work is to identify and quantify the effects of the number and type of repairs, the distance between them, and the pressurization of the pipe to sleeve gap on the mechanical behaviour of the component. The study includes full-scale experimental testing and finite element modelling. Burst tests were carried out in tracts of pipelines removed from service, including various geometric configurations with and without circumferential girth welds. It is concluded that the reliability of the repairs is strongly influenced by the construction procedures and that interaction effects between successive repairs are not appreciable if the repairs are more than a half pipe diameter apart.

Minimum thickness for circumferential sleeve repair fillet welds in corroded gas pipelines

The minimum weldable pipe wall thickness for sleeve repair welds is numerically assessed in this work, as a function of pressure during the welding operations of a corroded gas pipeline, according to the approach by Battelle. The minimum weldable thickness is found to increase when the flow rate of the transported gas in the section being repaired increases. Integrity of the repairs is assessed, and alternative measures to momentarily increase the flow in the area of the repair are evaluated.

Local collapse of gas pipelines under sleeve repairs

Abstract Local collapse of the pipe wall under full encirclement sleeve reinforcements is associated with breaks and blow outs that cause large gas losses and abrupt depressurisation in gas pipelines. Although these defects do not represent an imminent risk of failure, they should be eliminated because they impede the normal passage of the “instrumented pig” for internal inspection. Four failed repairs were experimentally evaluated, and the effects of different geometric factors were numerically assessed via non-linear numerical modelling of fluid flow and pipe response. All possible causes of the appearance of these defects and measures to minimise their occurrence were evaluated. The position of the repaired portion with respect to the blow out, local geometry of the repair and previous defects, and the amount of gas caught in the interstice between the pipe and the reinforcement, have an important part in the event. The measures for the prevention of this problem involve the use of fillers and improved construction of repair sleeves.

A simple lefm method for the contact problem in partially closed cracks

Abstract A new method is developed to study the contact problem in cracks subjected to a strong gradient in the surface direction of the crack, with compressive loads acting on part of it. The approach is based in geometrical considerations and uses the Weight Function method to obtain the effective crack length and mode I crack tip stress intensity factor. The method is illustrated for an infinite cracked plate with linear and quadratic distributions of a monotonically increasing load. Excellent agreement between analytical and numerical (FEM) results obtained. Possible applications in three-dimensional cracks are also discussed.

Application of a weight function based method to assess partially closed cracks

Abstract A new method to study the contact problem in cracks subjected to a strong gradient in the surface direction was presented in a previous work. The method uses the weight function corresponding to the geometry produced by the partial closure of the crack, and is useful to solve the case of an arbitrary monotonically increasing load applied to an infinite cracked plate. In the present work, we study the cases of centre-cracked strips and three-dimensional embedded circular cracks subjected to different load configurations. Weight functions provided by handbooks as well as a geometrically defined weight function are evaluated and used to solve these problems. A modified weight function is presented for the centre-cracked. strip. All results are compared with those obtained using Finite Element and Boundary Element Methods, with good agreement.

Flaw analyses of vintage LPG carafes in use in Argentine households

Abstract This paper discusses the failure of a vintage 10 kg LPG balloon or carafe, manufactured in 1968, and the following experimental study of material properties and fabrication procedures of other vessels of similar age. It is concluded that many of the carafes present fabrication and in-service defects, especially in the circumferential welds, that can become critical. Continuing use of these vessels is potentially dangerous, especially when the effectiveness of the post weld stress relief treatment can not be guaranteed.

Failures and integrity of pipelines subjected to soil movements

Buried pipelines interact with the soil that is founding them. Pipelines are subjected to additional loads transmitted by the movement of the ground. Geotechnical remedial works to control soil (fast) slides or (slow) creeping are often costly, so the proper placement of sensors and monitoring of deformations is an important variable to decide an intervention for stress relief. To this purpose, the key tools are nonlinear soil-pipe computer simulation, geotechnical surveys, and strain monitoring, an essential tool for keeping the integrity of pipelines buried in potentially unstable soils. Vibrating wire strain gages are frequently used to assess the axial and bending loads transmitted to the pipe. Limit values are set as alarm levels to indicate when to carry out remedial works and/or stress relief.